Water with depth adds lots of possibilities, mainly in three categories: Construction costs, construction restrictions, and ship movement.
The first thing I did was make a simple construction restriction, only allowing ship depots to be built at shallow depths, which typically means near the shore. That's pretty much just a proof of concept, I don't know if that's interesting at all, maybe it's not worth the effort to have in.
Next was making the cost of clearing water tiles depend on depth, squared. Depth is a 4 bit value ranging from 0 to 15, with 0 being "shallow" and 15 something like 300 m deep or more. By multiplying the cost of demolishing water by the square of depth, clearing the deepest water is 225x more expensive. On the "low" construction costs setting, that means a little over £7200 for clearing water near the shore, but almost £1.7 million for clearing deep ocean.
This makes large bodies of water much less attractive to fill in and build land bridges across, even making a few small artificial islands for bridges can cost many millions. Long bridge segments, taking the long way around, or transferring to ships can become the more attractive options with this.
More possibilities include letting NewGRF industries query water depth, so some industries can require certain depths to be built.
Right now there is some logic in the water tile loop to make water depth adjust over time, forming an even gradient out from the coast. I'd rather not have that, since it will also smooth out any intentional depth choices made by map authors. If anything, it should take decades or centuries for meaningful changes to happen. Right now it's mostly there to cause depth to happen at all, since there aren't any scenario editor tools to change it.
- Drawing: New water tile sprites representing different depth levels would be a great help for players to make choices.
I would really appreciate if someone can paint new water tile variations that somehow show that the water is deeper or shallower.
- Code: Terrain generators need to be reworked to account for depth and generate interesting water bodies.
In particular, they should be able to make shallow lakes, deep lakes, shelves of shallow areas near coast, sudden drops into much deeper ocean, and possibly do more interesting things with rivers.